(1) Field of the Invention
The invention relates to a method of producing an actuator of the type suitable for use in a fuel injection system for internal combustion engines. The method involves poling a ferroelectric sample so as to induce bulk piezoelectricity, and then implementing the sample in the actuator.
(2) Description of Related Art
In a piezoelectric crystal, mechanical strain or force is generated when an electric field is applied across opposite faces of the crystal. When the applied field is removed, the crystal structure will return to its original shape. Inorganic materials are only naturally piezoelectric in their single crystal form, whereas in a polycrystalline sample the individual crystallites are usually randomly oriented following manufacture. Although the crystallites individually exhibit piezoelectric coupling, the lack of overall preferred orientation means a piezoelectric effect is not apparent in the bulk material. Before such polycrystalline samples can be used in the manufacture of actuators, for example, it is therefore necessary to pole the material to align the dipoles, so as to give a crystal lattice structure with a preferred axis and direction.
In a ferroelectric crystal, permanent crystallographic reorientation can be induced by applying a sufficiently large electric field across opposite faces of the crystal (i.e. poling the crystal). Ferroelectric poling is illustrated in FIG. 1, which shows electric dipole moments 10 in a ferroelectric polycrystalline sample 12 (a) before poling (b) during poling and (c) after poling. The minimum electric field strength necessary to cause permanent crystallographic realignment, and dipole reorientation, is referred to as the “coercive” field strength. Once the coercive field strength is exceeded, the ferroelectric dipoles 10 become aligned (FIG. 1(c)). The permanent crystallographic reorientation that takes place during poling causes a small but significant change in sample shape, comprising an elongation along the field axis and a constriction normal to it. This is referred to as “ferroelectric strain”. After the aforementioned crystallographic reorientation takes place, the material will exhibit bulk piezoelectricity. In the poled state, the application of a further applied field will induce a shape change, known as piezoelectric strain, (as described previously). This lasts only whilst the electric field is applied.
Referring to FIG. 2, if the further electric field applied to the ferroelectric sample 12 is of the same polarity as the poling field, the piezoelectric material dipole 10a is extended along the poling axis. In such circumstances the piezoelectric material extends along the field axis and constricts normal to the field axis (see FIG. 2(a)). If the further electric applied field is of opposite polarity to the poling field, the piezoelectric dipole 10b is caused to contract along the poling axis. In such circumstances the piezoelectric material constricts along the field axis and dilates normal to the field axis (see FIG. 2(b)).
The inducement of piezoelectric strain is utilized in piezoelectric actuators, such as those used in fuel injection systems. Such actuators typically include a stack of piezoelectric elements across which an electric field is applied to cause contraction or extension of the piezoelectric sample, in use. Extension (or contraction) of the stack is used to apply an actuation force directly or indirectly to a mechanical component, for example a valve element, for the purpose of controlling fuel injection. An injector of the aforementioned type is described in our co-pending patent application EP 0995 901 A1.
If ferroelectric samples of significant thickness are to be poled, relatively high poling voltages are required. Poling such materials requires voltages of the order of 1-2 kV/mm, and so even a sample having a thickness of just 1 centimeter requires application of between 10 and 20 kV.
Similar field strengths are also often required with piezoelectric samples to produce the levels of extension and contraction required in fuel injection system actuators. Such high voltages require expensive supply electronics, and it is undesirable to use such high voltages in a vehicle.
Multilayer ferroelectric samples with bulk piezoelectricity have been developed in order to reduce the poling voltages required. As shown in FIG. 3, a multilayer structure 14 is formed from a plurality of relatively thin piezoelectric layers 16, each of which is spaced from its adjacent layers by an internal electrode of a group of internal electrodes, 18a or 18b. 
Alternate ones of the internal electrodes are grouped together to form the two sets of electrodes 18a, 18b such that the electrodes of one set 18a are interdigitated with the electrodes of the other set 18b. The internal electrodes of each set are connected together by means of first and second external electrodes 20a, 20b. By creating such a multilayer structure, for example having an electrode spacing of around 100 microns, an electric field of around 2 kV/mm can be achieved from an applied voltage of 200V. Due to the alternating polarity of the internal electric field, the poling direction in the piezoelectric multilayer alternates throughout the structure, as shown in FIG. 4.
Although the use of multilayer structures enables reduced poling voltages to be used, problems exist with the electrode designs which achieve this. Firstly, the two interdigitated sets 18a, 18b of internal electrodes must be isolated from each other to prevent short-circuiting, and to enable an electric field to be applied across the intervening material. Isolation of the interdigitated electrodes is usually achieved by terminating the electrode layers short of the opposite polarity external electrode (i.e. internal electrodes 18a terminate short of the external electrode 20b, and vice versa). However, referring to FIG. 5, a ferroelectric can only be poled where it is exposed to the coercive electric field and incomplete internal electrode layers therefore create regions 22 (shown hatched) of unpoled material adjacent to the external electrodes 20a, 20b. This gives rise to a ferroelectric strain discontinuity between the regions of poled and unpoled material, placing the unpoled material in tension and the poled material in compression. As a result, the poled material has a tendency to fracture and the unpoled material tends to clamp the adjacent poled material, thereby causing the multilayer structure to distort.
Secondly, in order to avoid undesirable “surface flashover” effects, which arise if the internal electrodes 18a, 18b meet the free surface of the sample, the internal electrodes 18a, 18b are buried by terminating them short of the free surface. Again, this leads to a region of the ceramic that remains unpoled (i.e. a region immediately below the free surface, typically having a width of a few hundred microns), resulting in ferroelectric strain discontinuities between poled and unpoled regions.
One known way of preventing surface flashover whilst avoiding the use of buried internal electrodes is to apply some form of passivation to the surface of the sample, such as a polymer encapsulation with relatively high dielectric strength. This provides a partial solution, but an unpoled region of the ceramic remains behind the external electrodes 20a, 20b to isolate the two sets of internal electrodes 18a and 18b. The outermost ones of the piezoelectric layers 40a, 40b that define the end faces of the multilayer structure 14 do not lie between two internal electrodes, and so are not exposed to the coercive field during poling. The outermost piezoelectric layers 40a, 40b therefore remain unpoled.
It is known to apply a poling voltage to a multilayer ferroelectric block in order to induce bulk piezoelectricity in the block. Following poling, the block is then divided or cut into individual samples. For example, U.S. Pat. No. 6,356,008 describes a method of producing a resonator device having a piezoelectric body in which a two stage poling method is applied to a ferroelectric block to induce piezoelectricity. Electrodes are then removed from the block and the block is cut to size to produce the final resonator device.